The present invention relates to an apparatus and process for forming meltblown fibers. More specifically, the present invention relates to an apparatus and process for forming meltblown fibers utilizing an extended jet thermal core produced by entraining hot air at the point of jet thermal core formation.
Meltblown fibers are fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging, usually hot and high velocity, gas, e.g. air, streams to attenuate the filaments of molten thermoplastic material and form fibers. During the meltblowing process, the diameter of the molten filaments are reduced by the drawing air to a desired size. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. Nos. 3,849,241 to Buntin et al., 4,526,733 to Lau, and 5,160,746 to Dodge, II et al., all of which are hereby incorporated herein by this reference. Meltblown fibers may be continuous or discontinuous and are generally smaller than ten microns in average diameter.
In a conventional meltblowing process, molten polymer is provided to a die that is disposed between a pair of air plates that form a primary air nozzle. Standard meltblown equipment includes a die tip with a single row of capillaries along a knife edge. Typical die tips have approximately 30 capillary exit holes per linear inch of die width. The die tip is typically a 60xc2x0 wedge-shaped block converging at the knife edge at the point where the capillaries are located. The air plates in many known meltblowing nozzles are mounted in a recessed configuration such that the tip of the die is set back from the primary air nozzle. However, air plates in some nozzles are mounted in a flush configuration where the air plate ends are in the same horizontal plane as the die tip; in other nozzles the die tip is in a protruding or xe2x80x9cstick-outxe2x80x9d configuration so that the tip of the die extends past the ends of the air plates. Moreover, as disclosed in U.S. Pat. No. 5,160,746 to Dodge II et al., more than one air flow stream can be provided for use in the nozzle.
In some known configurations of meltblowing nozzles, hot air is provided through the primary air nozzle formed on each side of the die tip. The hot air heats the die and thus prevents the die from freezing as the molten polymer exits and cools. In this way the die is prevented from becoming clogged with solidifying polymer. The hot air also draws, or attenuates, the melt into fibers. Other schemes for preventing freezing of the die, such as that detailed in U.S. Pat. No. 5,196,207 to Koenig, using heated gas to maintain polymer temperature in the reservoir, are also known. Secondary, or quenching, air at temperatures above ambient is known to be provided through the die head, as in U.S. Pat. No. 6,001,303 to Haynes et al.
Primary hot air flow rates typically range from about 20 to 24 standard cubic feet per minute per inch of die width (SCFM/in).
Primary air pressure typically ranges from 5 to 10 pounds per square inch gauge (psig) at a point in the die head just prior to exit. Primary air temperature typically ranges from 450xc2x0 to 600xc2x0 Fahrenheit (F), but temperatures of 750xc2x0 F. are not uncommon. The particular temperature of the primary hot air flow will depend on the particular polymer being drawn as well as other characteristics desired in the meltblown web.
Expressed in terms of the amount of polymer material flowing per inch of the die per unit of time, polymer throughput is typically 0.5 to 1.25 grams per hole per minute (ghm). Thus, for a die having 30 holes per inch, polymer throughput is typically about 2 to 5 lbs/inch/hour (PIH).
Moreover, in order to form meltblown fibers from an input of about five pounds per inch per hour of the polymer melt, about one hundred pounds per inch per hour of hot air is required to draw or attenuate the melt into discrete fibers. This drawing air must be heated to a temperature on the order of 400-600xc2x0 F. in order to maintain proper heat to the die tip.
Because such high temperatures must be used, a substantial amount of heat is typically removed from the fibers in order to quench, or solidify, the fibers leaving the die orifice. Cold gases, such as air, have been used to accelerate cooling and solidification of the meltblown fibers. In particular, in U.S. Pat. No. 5,075,068 to Milligan et al. and U.S. Pat. No. 5,080,569 to Gubernick et al., secondary air flowing in a cross-flow perpendicular, or 90xc2x0, direction relative to the direction of fiber elongation, has been used to quench meltblown fibers and produce smaller diameter fibers. In addition, U.S. Pat. No. 5,607,701 to Allen et al., uses a cooler pressurized quench air that fills chamber 71 and results in faster cooling and solidification of the fibers. In U.S. Pat. No. 4,112,159 to Pall, a cold air flow is used to attenuate the fibers when it is desired to decrease the attenuation of the fibers.
Through the control of air and die tip temperatures, air pressure, and polymer feed rate, the diameter of the fiber formed during the meltblown process may be regulated. For example, typical meltblown polypropylene fibers have a diameter of 3 to 4 microns.
After cooling, the fibers are collected to form a nonwoven web. In particular, the fibers are collected on a forming web that comprises a moving mesh screen or belt located below the die tip. In order to provide enough space beneath the die tip for fiber forming, attenuation and cooling, forming distances of at least about 8 to 12 inches between the polymer die tip and the top of the mesh screen are required in the typical meltblowing process.
However, forming distances as low as 4 inches are described in U.S. Pat. No. 4,526,733 to Lau (hereafter the Lau patent). As described in Example 3 of the Lau patent, the shorter forming distances are achieved with attenuating air flows of at least 100xc2x0 F. cooler than the temperature of the molten polymer. For example, Lau discloses the use of attenuating air at 150xc2x0 F. for polypropylene melt at a temperature of 511xc2x0 F. to allow a forming distance between die tip and forming belt of 4 inches. The Lau patent incorporates passive air gaps 36 (shown in FIG. 4 of Lau) to insulate the die tip.
Past efforts have largely focused on improved quenching in these short distances, where it can take as little as 1.3 ms for the meltblown extrudate to travel from the die to the collecting wire. The present invention approaches the problem of meltblown fiber formation from the opposite direction by seeking to increase the dwell time of the extrudate within the hot jet thermal core in order to further attenuate the fibers and also to allow the fibers to be formed from lower viscosity resins than were previously practical.
The present invention provides a method for producing super fine meltblown fibers by increasing the length of the meltblown jet thermal core to increase the dwell time of the extruded thermoplastic polymer within the jet thermal core. Through use of the method it is practical to use low viscosity resins and further to provide the resultant meltblown nonwovens with superior barrier properties to the passage of fluids and particularly gases.
The apparatus for practicing the method is both economical and easily retrofitted to existing meltblown fiber apparatus.
In essence, an entrainment duct or heat source is placed at the point of formation of the jet thermal core (hereinafter sometimes referred to synonymously as xe2x80x9cjetxe2x80x9d) and used to shroud the jet area from cold air and entrain warm air into the jet thereby lengthening it. Thus, the jet will provide higher temperatures over a longer distance and time for the extruded fibers and maintain a low melt viscosity during the fibers"" passage through the fiber attenuation zone.
Through the use of the lengthened jet, lower viscosity resins than heretofore practical may be used to form the fibers. Further, the resultant web of fibers made according to the present invention will have superior barrier properties to the passage of air and other fluids making a useful fabric for either barrier or filtration applications. Also, due to increased jet length, polymer additives may tend to bloom towards the surface of the fibers. Practical applications of fabric made according to the present invention may include barrier fabrics such as surgical gowns or the like and filtration materials.